Had the same issue, and was able to resolve it by replacing the line which queries my (only) Main-Menue:
‘<?php wp_nav_menu(‘title_li=’); ?>’
to:
`<?php wp_nav_menu( array( ‘title_li=’ => ”, ‘theme_location’ => ‘my-main-menue’ ) ); ?>’
‘my-main-menue’ should be replaced by the name of the Menu which is registered in your functions.php
